Bierlein U.S. Pat. No. 3.949.323 discloses the preparation of crystals, such as KTiO(PO.sub.4) crystals (KIT crystals), that are useful as second harmonic generators (SHG).
Miyazuka et al. U.S. Pat. Nos. 4,953,931 and 4,953,943 describe nonlinear devices having a LiNbO.sub.3 thin film waveguide layer formed on a LiTaO.sub.3 substrate.
Tamada et al. U.S. Pat. No. 5,022,729 discloses a SHG having a Ta.sub.2 O.sub.3 TiO.sub.2 system amorphous thin film optical waveguide on a substrate. The substrate may be a nonlinear optical crystal material. The waveguide can be made more effective by forming periodically poled regions of selected period and depth in the nonlinear substrate.
Yamamoto et al. U.S. Pat. No. 4,591,291 discloses a semiconductor laser and an optical nonlinear device positioned on a submount with the laser's active layer and the nonlinear device's surface waveguide facing the submount so that the fundamental light from the laser is directly applied to the nonlinear device and doubled in frequency producing a visible light
Yamamota et al. U.S. Pat. No. 5,253,259 discloses a frequency doubler comprising a nonlinear crystal having domain inverted regions and a waveguide coupled to a semiconductor diode laser. The device includes a means for heating the frequency doubler to tune it to the desired frequency regardless of the ambient temperature.
Endo et al. U.S. Pat. No 5,546,220 discloses an optical structure that is clad with a metal coating to conduct electricity and uniformly heat an optical structure to tune it.
Welch U.S. Pat. No. 5,185,752 discloses a diode laser having a reflective back end coupled to a SHG. The SHG comprises a periodically poled waveguide having ferroelectric domains and a periodic reflector, particularly a distributed Bragg reflector (DBR) grating. This arrangement forms an optically resonant chamber feedback system that stabilizes the frequency output of the diode laser and efficiently couples the diode laser to the SHG.
By the techniques described above, and numerous others, significant improvement has been made in the efficiency, in terms of power output, currently being obtained from laser/SHG systems. Nevertheless, the efficiencies obtainable by the currently available devices are still very low. There is a need for achieving far greater efficiency, and so output power, because higher power laser/SHG units would be useful in a number of areas where current devices are unsatisfactory, such as optical data storage, remote sensing, and therapeutic medical applications. Practicing the present invention, using a KTP crystal optical structure, increases the output power from 2-5 times that from a similar KTP optical structure used in accordance with the prior art. Also the narrower wavelength of the output light from the devices of the present invention makes them more accurate in devices for detecting specific materials, avoiding false readings and making it unnecessary to have expensive devices to avoid false readings.
It has been observed that the second harmonic efficiency of nonlinear harmonic generator crystals of identical dimensions and apparent compositions often have different second harmonic generation efficiencies. It is also believed that various locations along the length of a single crystal may have different generation efficiencies. Also it has been observed that during second harmonic generation the generator tends to heat differentially along the length of the generator, with the greatest heating occurring in the zone near the output end of he generator. Additionally it has been observed that the internal conversion efficiencies of longer crystal generators tends to be less than for shorter length generators. The internal conversion efficiency, .eta. in % is defined as: ##EQU1## where P.beta..gamma.P.beta.w.gamma. are the power of the input fundamental and second harmonic beams respectively and l is the length of the waveguide.
The method of the present invention increases the internal conversion efficiency of nonlinear optical structures.